1,2) public member function, calls the protected virtual member function do_is of the most derived class.

3) Checks if the character c is classified by the mask m

4) For every character in the character array [low, high), identifies the complete classification mask (e.g. digit|xdigit|alnum|print|graph for the digit '0' in the default locale), and stores the masks in the corresponding elements of the array pointed to by vec